Macbook won't turn on unless I hit it, and it freezes if moved around. Any ideas?

My wife's had a 13" macbook for several years, and it's always had a propensity for freezing up if she moves it around a lot. Shortly after the warranty period expired, it began to refuse to boot up unless she firmly tapped the case, and the problems with freezing increased.

It's all factory condition, never been opened up or messed with. If I had some idea of where the problem was, it would really help me plan my repair efforts.

We haven't noticed any hard drive problems or other quirks.